The present invention relates to a sifting device.
In known sifting devices of this type, sieve mats are used, which are alternately tensioned and relaxed in sections over the sieve surface. This is performed mechanically, for example, two movably implemented oscillation systems, such as sieve boxes, being set in oscillation in relation to one another by an eccentric drive using transverse carriers. The plastic sieve liner strips are connected in this case on one end to the transverse carriers of the first sieve box and, on the other end, to the transverse carriers of the second sieve box. Through the opposing oscillations of the two sieve boxes in relation to one another, and therefore of the particular transverse carriers, the plastic sieve liner strips are alternately tensioned and relaxed. In this way, the product to be sifted is separated into the individual fractions. Through the continuous tensioning and releasing, the sifting covers clean themselves again and again and good separation is possible even with sifting product which is difficult to sift.
However, since the sifting product is not subjected to any acceleration in the conveyance direction of the sifting machine through the oscillation, these sifting machines are also placed at a relatively large slant, so that the sifting product may be conveyed by gravity.
Other known sifting machines include a sieve box having transverse carriers to which an oscillating frame having further transverse carriers is elastically coupled. In these sifting machines, the individual sieve liner strips are connected on one end to the transverse carriers of the sieve box and on the other end to the transverse carriers of the oscillating frame. The sieve box is typically caused to oscillate by being excited by the force of an unbalanced drive and may be referred to as a fundamental oscillation system. Through the oscillation of the sieve box, the oscillating frame elastically coupled to this sieve box is caused to oscillate with corresponding tuning of its oscillation system (mass-spring), and thus represents a supplementary oscillation system. Through the oscillation of the sieve box, the product to be sifted is also subjected to acceleration in the conveyance direction of the sifting machine, so that the product is also conveyed solely by the machine. Slanted placement of the sifting machine is therefore not absolutely necessary, and/or only a significantly lesser slant is necessary.
Through the alternating tensioning and releasing of the sieve mats, which are installed in such sifting devices over the sifting length, these mats clean themselves again and again from jammed grain and caking which is caused by fine, damp sifting product.
Banana sieves represent another embodiment. This refers to sifting devices in which the sieve surface is divided into individual sieve surface sections over the sifting length, each of these sieve surface sections having a different slant within the sifting machine. Such constructions are preferably selected if larger feed quantities are to be separated according to grain size and the cutpoint is relatively close to the largest grain of the feed product. The advantage of such sifting machines is that it is possible to keep the dumping height of the sifting product essentially the same over the entire length of the sifting machine through the. differing slants and thus the different conveyance speeds of the product to be sifted, although there is less and less material to be conveyed and sifted on the sifting plane due to the sifting and the passage of the fine fraction through the screen fabric. Uniform dumping height of the sifting product is of great advantage for effective sifting, since otherwise the particles close to the cutpoint only jump on the sifting plane toward the end of the sifting machine and do not have the possibility of making their way through a mesh of the screen fabric.
Sifting machines of this kind are known from, e.g., EP 0 197 191, EP 1 142 651 and WO 92/00148.